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A thermobaric weapon is an explosive weapon which uses an initial
small explosion to disperse reactive material over a relatively
large volume. Subsequent heat-producing reaction of this material
generates a pressure wave that is responsible for much of the
weapon's destructive effect.

Thermobaric explosives are highly effective in confined spaces such
as tunnels, caves or underground bunkers. Rather than providing
protection as they would from conventional explosive ammunition,
structural interior walls, particularly cement or other hard
surfaces, magnify and channel the shock waves created by a
thermobaric detonation. The stronger the walls, the higher the
pressure’s reflective effect.The turbulent mixing of fuel with
ambient oxygen is induced by the presence of walls through enhanced
mixing from three different types of instabilities as well as from
enhanced chemistry from temperature and pressure velocity gradient
in differing fuels, creating a piston type afterburn reaction in
enclosed structures.

Variants

Thermobaric weapons include fuel-air explosives (FAE or
FAX) in which the dispersed material is fuel, and the
heat-producing reaction is combustion with atmospheric oxygen. In other cases (metal-augmented
charge) the dispersed material contains powdered metals such
as aluminum or magnesium, which generate heat by burning in air, or
in the gaseous reaction products of the initial explosion, or with
the help of oxidizers included in the dispersed powder. Other terms
used for this family of weapons are high-impulse thermobaric
weapons (HITs), heat and pressure weapons, or
vacuum bombs. Thermobaric weapons that depend on
atmospheric oxygen can produce more explosive energy for a given
size than conventional explosives, but have the disadvantage of
being less predictable, being influenced by weather.

Mechanism

A thermobaric explosive consists of a container of a finely
powdered solid fuel of differing particle size mixed with a low
percentage of oxidizer and binder. The solid fuel could be an
explosive metal powder or reactive organic. A high explosive charge
is placed in the middle of the mixture.

A thermobaric weapon is initiated upon dropping or firing, and the
explosive charge (or some other dispersal mechanism) bursts open
the container and disperses the fuel in a cloud, and ignites the
mixture in a single event. The heat released by the oxidizer gases
then helps ignite the smaller solid particles that are mixed with
the compressed hot air behind the shock, which leads the blast
wave. This sustains a hot environment which allows 100% fuel
combustion to be achieved. If fuel particles have a size
distribution, smaller particles are quickly ignited, providing heat
for the combustion of the larger particles. Smaller particles burn
rapidly and remain tied to the local gas, while the larger
particles move more freely and mix with new oxidation sources,
allowing a more sustained combustion than would be produced by
particles of a single size.

In confined spaces, transition to full detonation is not required for enhanced blast, if
the solid fuel is ignited early in the dispersion process. A series
of reflective shock waves generated by the detonation mixes the hot
detonation gases with metal particles and compresses the metal
particles at the same time. These actions provide the chemical
kinetic support to maintain a hot environment, causing more metal
to ignite and burn. This late time metal combustion process
produces a significant pressure rise over a longer time duration
(10–50 msec). This is a phase generally referred to as after
burning or late-time impulse which can occur outside of where the
detonation occurred, resulting in more widespread damage.

This is an aerobic reaction that draws in all of the unburnt fuel
and atmospheric air, and creates a vacuum in the detonation
environment, giving rise to the informal term 'vacuum bomb'. An air
shock wave, generated during the fireball expansion, is reflected
from the walls of the structure. The reflected shock plays two
important roles. First, it stops the temperature decrease of the
air and the fireball. It can actually increase the temperature in
some places, depending on how the shock waves reflect. Second, it
creates two new types of flow instabilities; Richtmyer-Meshkov and Kelvin-Helmholtz
instabilities.

Weapon effects

Fuel-air explosives represent the military application of the vapor
cloud explosion and dust explosion accidents that have long
bedeviled a variety of industries. An accidental fuel-air explosion
may occur as a result of a boiling liquid
expanding vapor explosion (BLEVE), for
example when a tank containing liquefied petroleum gas bursts.
Silo explosions, caused by the
ignition of finely-powdered atmospheric dust,
are another example.

The detonation of thermobaric explosives (TBX) can be viewed in
three stages. The first, an anaerobic stage, is measured in
microseconds and breaks down the explosive by a shock wave. The
subsequent exothermic molecular reactions go on to propagate the
detonation wave. The second stage, measured in hundreds of
microseconds, is also anaerobic. This involves reactions between
any products that were too large to be involved in the main
detonation event. The third stage is aerobic and lasts
milliseconds. In this stage more, previously unreacted, fuel
particles react with the surrounding air.

Stage One defines the HE's high-pressure shock effects (such as
propelling a metal liner or fragments); Stage Two prolongs the
high-pressure blast pulse, giving a useful heaving effect needed in
building or bunker defeat; and Stage Three produces a
long-duration, lower-pressure pulse that can also have a high
thermal output, both of which are useful for materiel and personnel
defeat.

Stages Two and Three are enhanced in thermobarics. This is
accomplished by the addition of various fuels and additional
oxygen-carrying chemicals to the explosive. The fuel is normally
finely powdered aluminium, but boron, silicon, titanium, magnesium,
zirconium, carbon and hydrocarbons can also be used. A typical
oxygen-carrying chemical would be ammonium perchlorate. By
carefully selecting the HE, fuel and oxidiser, the multiple-target
defeat effects of blast, fragmentation and thermal pulse can be
brought into effect.

Blast enhancement is mainly due to two reasons. The first is the
fact of the wide dispersion of the fuel before combustion, making
the initial combustion zone very large in comparison with a
standard high explosive (metres compared with millimetres). The
second is that although the peak pressure produced is lower, the
duration is far longer. This is effective as the ability of
buildings and people to survive a given pulse pressure level
decreases with increasing pulse duration. The thermal effects of
such warheads also dwarf those of classical HE, the temperature of
the fireball, the heat flux produced and its duration all being
several times larger (some an order of magnitude greater).

Calculations

For vapor cloud explosion there is a minimum ratio of fuel vapor to
air below which ignition will not occur.
There is also a maximum ratio of fuel vapor to air, above which
ignition will not occur. These limits are termed the lower and
upper explosive limits. For gasoline vapor, the explosive range is
from 1.3 to 6.0% vapor to air, and for methane this range is 5 to
15%. Many parameters contribute to the potential damage from a
vapor cloud explosion, including the mass and type of material
released, the strength of ignition source, the nature of the
release event (e.g., turbulent jet release), and turbulence induced in the cloud (e.g., from
ambient obstructions).

The overpressure within the detonation can reach 430 lbf/in²
(3 MPa, 30 bar) and the temperature can be . Outside the
cloud the blast wave travels at over 2 mi/s (3 km/s).
Following the initial blast (compression) is a phase in which the pressure
drops below atmospheric pressure (rarefaction) creating an airflow back to the
center of the explosion strong enough to
lift and throw a human. It draws in the unexploded burning fuel to
create almost complete penetration of all non-airtight objects within the blast radius, which are then incinerated.
Asphyxiation and internal damage can
also occur to personnel outside the highest blast effect zone, e.g.
in deeper tunnels, as a result of the blast
wave, the heat, or the following air draw.

Calculations of enhanced blast explosives(EBX) are based on optical
pyrometry of the pyrophoric metals to determine combustion
temperature and rate. Depending on the metal particle size,
different combustion behaviour can be observed in the detonation
products: 315 µm particles present a delayed ignition with low
and short emission, while 5 µm particles react almost
instantaneously and keep burning for more than 40ms. The presence
of AlO at different times indicatesthat aluminium combustion occurs
with different delays depending on the particle size and
non-monotonousrates during the fireball expansion. By recording the
light spectrum emitted by metallized explosives, it is possible to
collect information on thepresence of certain species during the
fireball expansion. An average apparent temperature can also
bedetermined at each integration step, using the classic method of
the two-colour pyrometry. Although this technique can generate
significant errors in certain conditions, it does not require the
determination of emissivity of the observed area. This variable is
indeed hardly accessible since it depends on the wavelengthand the
chemical species present in the observed area. Previous studies
determined the temperature of metallizedexplosive fireball using
fixed wavelengths with better time resolution. The ISL spectroscope
allowschoosing any pair of two wavelengths out of any specific
atomic or molecular emission since all spectra are fullyrecorded
during the explosion duration. The two wavelengths chosen for this
study are 440 nm and 630 nm,corresponding to the apparent
grey emission zones of the spectrum and being in a similar
sensitivity range of thespectroscope sensor. Figure 5 presents the
estimated fireball temperature evolution during the explosion of
four2 kg charges for different aluminium particle sizes (5,
10, 100 and 315 µm).For homogeneous charges, the apparent
temperature of burnt products stagnates at approximately
2500Kduring 15ms. In the case of a heterogeneous fireball produced
by aluminized charges, the measured temperaturereaches levels
between 3000 and 3500K, influenced by the flame temperature of
aluminium mixed with air(3400K). Nevertheless the temperature tends
to approach the value recorded with homogeneous explosive.

History

The first thermobaric explosions may have been the unintended
ignition of flour in flour mills, a
phenomenon known since medieval times. Such explosions are the
consequence of the rapid burning of a fine fuel (the flour),
suspended in air in a confined space.

The introduction of flamethrowers in
the trench warfare of World War I could
constitute the first use of a primitive "vacuum weapon", in that
they could suffocate people protected from the direct weapon
effects inside a pillbox or bunker .
Other such
effects were seen to occur in the firestorms that followed the
Allied bombing raids at Dresden and
elsewhere .

During
World War II the ignition of fuel vapour within partially empty
aviation fuel tanks caused massive explosions that led to the loss
of several carriers including HMS Dasher .

In 1944 the Germans proceeded with the development of a fuel-air
bomb, using 40% liquid oxygen mixed
with 60% dry brown coal powder. In a test of an
8 kg charge near Doberitz, trees were completely destroyed
within a 600 meter radius, with shock effects being felt as far
away as 2 km. This was believed to be the beginning of
fuel-air and thermobaric weapon development. The extent of the
described destruction radius is not plausible for the stated mass
of the charge.

In the
form that exists today, these devices (also called Fuel-Air
Munitions) were developed in the 1960s and used by the United States during the Vietnam War
to destroy VietCong tunnels , clear forest
for helicopter landing sites and to clear minefields . FAMs are in published
literature available to English-speaking readers by the
mid-1970s.

The
Soviet armed forces
extensively developed FAE weapons, including thermobaric warheads
for shoulder-launched RPG
(RPO-A ShmelBumblebee /
/).Russian forces have a wide array of these
weapons and used them against Chinese forces in the Sino-Soviet border conflict of
1969 , and used them in Afghanistan and in Chechnya.
Russian troops report that a single RPO-A round in an urban
environment has an equivalent effect to a 152 mm artillery
round . TOS-1 "Buratino"
is another Russian Army FAE weapon system, composed of a multiple rocket launcher mounted on
a T-72 chassis. The TOS-1 was the main thermobaric
delivery system that the Russians used against Grozny in the
Second Chechen War.

A FAE
system from Israel was
developed for minefield clearing .
The system uses a small rocket-propelled thermobaric charge which
explodes over the minefield and activates exposed or buried
mines.

Thermobaric and fuel-air explosives have been
used by terrorists since the 1983 Beirut
barracks bombing in Lebanon which used a gas-enhanced explosive
mechanism, probably propane, butane or acetylene.The
explosive used by the bombers in the 1993 World
Trade Center bombing was based on the FAE principle, using three tanks
of bottled hydrogen gas to enhance the
blast.In 2002, Jemaah
Islamiyah bombers used a shocked dispersed solid fuel charge,
based on the thermobaric principle, to attack the Sari nightclub in
the 2002 Bali
bombings.

In 2003, United States Marines used a thermobaric version of their
Shoulder-Launched
Multipurpose Assault Weapon, called a Shoulder-Launched
Multipurpose Assault Weapon-Novel Explosion (SMAW-NE), in the
Invasion of Iraq. One team of
Marines reported that they had destroyed a large one-story masonry
type building with one round from 100 yards. The thermobaric
explosive used in this weapon, PBXIH-135 or a variant, was
developed at the Naval Surface Warfare Center (NSWC) Indian Head
Division and had previously been used in BLU-118/B air-dropped
bombs against al Qaeda and Taliban forces in Afghanistan in early March,
2002.

Newest U.S. small arms FAE munitions

Introduced to the Afghanistan conflict, the XM1060 40-mm grenade is
perhaps the first small-arms thermobaric device released in a U.S.
theatre of war. Developed and fielded in just under five months by
the Picatinny Arsenal, the XM1060
was delivered to U.S. forces in Afghanistan on April 30, 2003. The
grenade was designed to be used with existing battlefield delivery
systems presently in use by squad-level field forces.

The 48-lb (22 kg) AGM-114N
Hellfire Metal Augmented Charge introduced in 2003 in Iraq
contains a thermobaric explosive fill, using fluoridatedaluminium
layered between the charge casing and a PBXN-112 explosive mixture.
When the PBXN-112 detonates, the aluminium mixture is dispersed and
rapidly burns. The resultant sustained high pressure is extremely
effective against enemy personnel and structures.

Russian test of the largest vacuum bomb

In September 2007 Russia successfully exploded the largest vacuum
bomb ever made, leveling a multi-story block of apartment buildings
with a power greater than that of the smallest dial-a-yield nuclear weapons at their lowest
settings. Russia named this particular ordnance the "Father of All Bombs" in response to the
United States developed "Massive Ordnance Air
Blast" (MOAB) bomb whose backronym is
the "Mother of All Bombs", and which previously held the accolade
of the most powerful non-nuclear weapon in history. The bomb
contains a 14,000 pound (6,400 kilogram) charge of a liquid fuel
such as ethylene oxide, mixed with an
energetic nanoparticle such as aluminium, surrounding a high explosive
burster.The FOAB is based on the Russian ODAB-500PM and the
BLU-82 Daisy Cutter. Shortly after the
announcement of the FOAB, the United States Air Force announced the
production of the 30,000 pound Massive Ordnance Penetrator,
utilizing a 6,000 pound thermobaric mixture encased in a 24,000
pound steel shell.

Afghanistan

In June
2008, the United
Kingdom revealed that its forces had used thermobaric
munitions in Afghanistan.
The munitions were delivered by the Hellfire AGM-114N from WAH-64 Apache attack helicopters. American
forces have also apparently been employing the weapons in
Afghanistan from Apaches and from unmanned drones. The UK stated
that the weapon will also be configured to be delivered from its
own MQ-9 Reaper drones.